Producing coatings on metal



Patented Nov. 8,1949

PRODUCING COATINGS ON METAL George R. Hoover and Norman Cox. Middletown,Ohio, assignors to Armco, Steel Corporation, a

corporation of Ohio No Drawing. Application September 1 0, 1947,

. Serial No. 753,30?"

This application is a continuation-in-part of application Serial No.437,414, having the same title, and filed April 2, 1942, which in turnis a continuation-in-part of our previous application, Serial No.415,080, filed October 15, 1941, under the same title, both of which arenow abandoned.

The invention relates to the production of phosphate coatings on metals,which coatings are effective either in rendering the surface of themetal passive to oxidation, or in providing a bond or tooth for thereception of paints, enamels, and the like, or both. In general,treatments for these purposes are set forth in such patents as Green2,082,950, Tanner et al. 1,888,189, Thompson et al. 1,869,121, Green1,651,694, and Thompson et al. 2,001,754. For convenience hereinafter weshall refer to the production of such coatings as ings of zinc and zincalloys and as applied to coatings which are high in or consist ofaluminum, than treatments hitherto found in the art.

It is an object of our invention to provide a Bonderizing method andsolution in which the activity of the phosphating solution is verygreatly stepped up, while the quality of the formed coating is fullymaintained. It is our object to produce coatings which are physicallystrong and tightly adherent, which are devoid of a coarse, granular orrough consistency, which are not soft and chalky, and which, whilefurnishing an excellent tooth for paints and enamels, are inert withrespect thereto. It is an object of our invention not only to provideBonderizing solutions of high activity, but solutions having uniformcoating producing effects, so that a better bond or tooth for paints andenamels can be provided with a thinner over-all coating.

Still more specifically, it is an object of our invention to provide avery substantial acceleration in the speed of action of Bonderizingsolutions by means of the addition of substances containing fluorine,without at the same time producing coatings containing significantamounts of fluorine, since it has been found that coatings containingfluorine have a detrimental effect upon superimposed coatings of paintsand enamels.

The objects of our invention which have been set forth above, and otherobjects which will be set forth hereinafter or will be apparent to the 4Claims. (0!. 1489-615) skilled worker in the art upon reading thesespecifications, we accomplish by that certain procedure, composition andtreatment, of which we shall describe an exemplary embodiment.

Our invention is applicable to usual Bonderizing solutions as previouslytaught in the art, modified by the additions hereinafter set forth. Itis also applicable to Bonderizing solutions departing in proportions ofingredients from those hitherto known.

In the development of the Bonderizing art, it was early found that asolution containing phosphoric acid or a phosphate could, under certaincircumstances, produce upon the surface of iron a passivating phosphatecoating. Such solutions were extremely slow; and it was ascertained thatthe addition of iron to the solution would accelerate its action. Th useof metals other than iron, such for example as nickel, copper,manganese, cobalt, and the like, were found to be helpful in speeding upand controlling the action; and the use of an oxidizing agent, such as anitrate, helped in the matter of control by tending to remove activehydrogen involved in the process. A content of zinc is alsoconventional, especially when Bonderizing galvanized products, and thezinc may be added, if desired, as zinc nitrate.

In general, the usual Bonderizing solutions are described in terms oftheir content of free acid and total acid (as determined by titrationwith different indicators) nitrate, zinc (or other metal-- licaccelerator) and iron, usually as ferrous iron.

The acid concentration in points is a measure arrived at by titrating a10 ml. sample of the solution with 0.1 normal sodium hydroxide. Thenumber of points is equal'to the number of milliliters of sodiumhydroxide necessary to produce thecolor change of the indicator. Theindicator used for determining free acid" was methyl orange, and thatused for determining total acid was phenolphthalein.

' A conventional Bonderizing solution had the following characteristics:

Total acid points 30 Free acid do 4 to (i Nitrate per cent 0.30 to 0.65Zinc do 0.30 to 0.60 Iron do 0.15 to 0.30

It was taught in Patent No. 1,869,121 that the ratio of free acid tototal acid should range between l:6 and 1:15 for best results.

An iron sheet or article was immersed in such a bath for a suflicientperiod of time and usually at an elevated temperature, after which itwas rapidly dried. The time duration of the treatment usually had to beof the order of several minutes. Such solutions were effective on thezinc surfaces of galvanized iron; but within commercial time limitationsthey were not satisfactorily effective on the surfaces of alloys of zincand aluminum, and on aluminum surfaces.

Hays and Hoover in Patent 2,346,302 taught that they could improve theaction of Bonderizing solutions on zinc surfaces, by making that actionmore uniform so that thinner coatings could be employed, and hencespeeding up the process, by increasing the nitrate content to between.65 and 1.10% and cutting down somewhat on both the free acid and totalacid. Acceptable coatings could be formed with immersions not exceedingtwo minutes, and in some instances as short as twenty seconds. y

In accordance with the present invention, we add to Bonderizingsolutions fluorine bearing compounds in small amounts, and in accordancewith the teachings hereinafter set forth, we achieve a marked and usefulacceleration of the action of the bath such that we are able to producecoatings in substantially less than one minute, and in instances in aslittle as five seconds or less. Coating produced in accordance with ourinvention contain less than 0.01% fluorine and do not affect the life ofpaints and enamels as shown by the standard accelerated paint-lifetests.

The nature of the fluorine bearing compounds is of the greatestimportance; and the compounds employed by us consist in hydrofluoricacid and its salts.

Complex fluorine bearing compounds such as the fluo-silicates,fluo-borates, fluo-titanates, fluo-vanadates, and the like should not beused. These are relatively very much less active as accelerators, andwhen used in proportions to provide useful acceleration, result incoatings which contain fluorine, and detrimentally affect the life'andadherence of coatings of paints and enamels. This is believed to be dueto the fact that when these substances ionize in solution, they formcomplex fluorine bearing ions which, joining with the base metal ormetals in the bath, go over into the coating. The use of such complexfluorine bearing compounds in conjunction.

with hydrofluoric acid or its salts is likewise ineffective, since thecoatings formed will be high in fluorine. In addition, the complexfluorine bearing compounds have a strong tendency to produce sludge inthe bath.

We employ substances capable of ionizing to yield the simple fluorineion, without the production of complex ions in which fluorine is joinedwith other elements. These conditions can be met, so far as we know,only through the use of hydrofluoric acid or its salts. In accordancewith our invention we employ baths containing from approximately 015% to.80% by weight of fluorine. These limits may be exceeded in eitherdirection, but below about 015% of fluorine, the acceleration falls offrapidly, while above about .80% the increase in acceleration is lessthan proportionally great and the formed coatings begin to pick upfluorine.

As indicated above, the fluorine maybe added in the form of hydrofluoricacid and its salts. Hydrofluoric acid has the effect of increasing boththe free acid and total acid in the bath.

Acid salts of hydrofluoric acid, such as ammonium bifluoride, alsoincrease the total and free acid content, but the latter to a lesserextent.

The normal salts of hydrofluoric acid, such for example as sodium orpotassium fluorides, or the fluorides of other metals or of ammonium,have the effect of increasing the total acid content of the bath, butnot its free acid content. The range of quantities of fluorine in thebath given above is easily calculated for any fluorine comfree to totalacid of from substantially 1:2 to

1:10, with a preferred range lying between 1:4 and 1:8. If the free acidis increased so that its ratio to total acid is 1 to less thanapproximately 2, the bath is likely to have an action in which picklingpredominates over coating. If the free acid is decreased so that theratio is l to more than 10, substantially, soft coatings are likely tobe produced, and the baths have an enhanced tendency to form sludges.Within the limits set forth, baths with high acid content tend to bemore vigorous and rapid in their action.

Ingeneral, we have found it advantageous to increase the content ofnitrate with an increase in the fluorine content. In other words, withinthe ranges set forth herein, the quantity of nitrate should be roughlyproportional to the quantity of fluorine. With an increase in thenitrate content, the iron in the bath appears to remain largelyferrous,'in the absence of external Free acid points 1.5 to 10.0 Totalacid I do 16.0 to Nitrate per cent .30 to 2.65 Zinc do .30 to 2 Ferrousiron, from less than do .002 to .30 Fluorine do .015 to .80

An exemplary solution for Bonderizing by dipping, and effective for thesheets or articles hereinafter set forth is as follows:

Free acid points 3 to 4 Total acid do 25 to 30 Nitrate per cent .90 to1.10 Zinc d0 .40 to .50 Ferrous iron do .08 to .25 Fluorine do .015 to.80

The materials to be Bonderized are treated with the solution at roomtemperature or above, but

perferably in the range of 120? to 210 F. for

lengths of time required to build up the desired weight of coating,which by reason of the enhanced uniformity of attack, may be a very thincoating. The time of immersion is brief, usually not above one minute asa maximum, and may be as short as five to fifteen seconds. The materialslined, are operative for iron, mild steel, galvanized iron or steel andmetals coated with zinc and its alloys and aluminum and its alloys.

The solutions will Bonderize chromium steels substantially free ofnickel.

The rapidity of the action'of our baths will vary somewhat with themetallic surfaces being treated. For'surfaces of aluminum and aluminumalloys high in aluminum, together with surfaces of iron, we prefer tooperate in the higher portions of the ranges set forth, i. e. withgreater amounts of free and total acids and greater amounts of fluorine.An alloy zinc with aluminum, where the aluminum content is small in'proportion to the zinc, is an intermediate material for which bestresults are obtained by operations in the midsections of the rangesgiven above. vanized materials, i. e. materials coated withsubstantially pure zinc, are easier to Bonderize and may, withadvantage, be treated in the lower portions of the ranges set forthabove.

Modifications may be made in our. invention without departing from thespirit of it.

Having thus described our invention, what we in the process andequivalent to 30% to 2.65%

nitrate, fluorine, and an acid phosphate .of a metal which will depositas a phosphate coating, water being the solvent, said solution having aratio of free acid to total acid of 1 :2 to 1:10, a free acid contentfrom substantially 1.5 to points, a total acid content vfromsubstantially 16.0 to 65 points, and in which the fluorine in thesolution results from the addition of a material from the classconsisting of hydrofluoric acid and its salts. the amount of fluorinepresent being substantially 0.015% to 0.80% by weight.

2. The solution of claim 1 in which the stated fluorine content resultsfrom the addition of from substantially 025% to substantially 1.2% ofammonium bifluoride.

3. An accelerated phosphate coating solution capable of yieldingsubstantially fluorine-free paint receiving coatings on iron, zinc andaluminum surfaces and consisting essentially of an oxidizing agentadapted to remove hydrogen in the process and'equivalent to 30% to 2.65%nitrate, fluorine, and an acid phosphate of zinc,

aiamav water being the solvent, said solution having a ratio of freeacid to total acid of 1:2 to 1:10, a free acid content fromsubstantially 1.5 to 10 points, a total acid content from substantially16.0 to 65 points, containing ferrous iron from substantially .002% to.30%, in which the zinc constitutes substantially .30 to 2%, and inwhich the fluorine in the solution results from the addition of amaterial from the class consisting'of hydrofloric acid and its salts,the amount of fluorine present being substantially 0.015% to 0.80%

Ordinary galby weight.

4. A process of producing substantially fluorinefree phosphate coatingon iron, zinc and aluminum surfaces, which comprises immersing articleshaving such surfaces in a solution consisting essentially of anoxidizing agent adapted to remove hydrogen in the process and equivalentto .30% to 2.65% nitrate, fluorine, and an acid phosphate of a metalwhich will deposit as a phosphate coating, water being the solvent, saidsolution having a ratio of free acid to total acid of 1:2 to 1:10, afree acid content from substantially 1.5 to 10 points, a total acidcontent from substanstially 16.0 to 65 points, and in which the fluorinein the solution results from the addition of a material from the classconsisting of hydrofluoric acid and its salts, the amount of fluorinepresent being substantially 0.015% to 0.80% by weight, maintaining saidsolution at temperatures between substantially 120 and substantially 210F., and withdrawing the articles from said solution after a treatmenttime of substantially 5 seconds to substantiaily 1 minute.

GEORGE R. HOOVER. NORMAN COX.

REFERENCES CITED The following references are of record in th file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,911,726 Tanner et al. May 30,1933 2,203,670 Buzzard June 11, 1940 2,234,206 Thompson Mar. 11, 19412,312,855 Thompson Mar. 2, 1943 2,314,565 Thompson Mar. 23, 1943 FOREIGNPATENTS 7 Number Country Date 522,681 Great Britain June 25, 1940728,364 France Apr. 11, 1932 728,411 France Apr. 12, 1932

